The Australian-Antarctic Discordance

		The Australian-Antarctic Discordance
1. Introduction 2. Tectonic History 3. Isotopic Boundary 4. Depth Anomaly 5. Mantle Anomaly 6. Plate Boundary 7. FAQs 8. Previous Research 9. Glossary 10. References		The Australian-Antarctic Didscordance (AAD) is defined as the deepest portion (~4-5km) of the global mid-ocean ridge (MOR) system (Gurnis et al., 1998). It is approximately 600km long and is a segment of the South East Indian Ridge (SEIR) south of Australia, between 115 and 128˚E (Figure 1.1; Gurnis & Müller, 2003). Figure 1.1 Location of the Australian-Antarctic Discordance. The north-south trending crenulations can be seen (Source: http://www.geosci.usyd.edu.au/research/marinegeophysics/Resprojects/geo_figures.html). The characteristics of the AAD include: Depth Anomalies The residual topography of the sea floor reveals a linear north-south-trending depression extending from the Great Australian Bight (GAB), through the region of the AAD, to the Antarctic margin, with residual topography values exceeding 1000 m (Figure 1.2; Gurnis et al. 1998; Gurnis & Müller, 2003). Gurnis et al. (1998) explain that in comparison with global residual oceanic topography, the AAD is a well-known bathymetric anomaly that is not associated with present-day subduction. This depth anomaly is symmetrical with respect to the SEIR which suggests that it is a property of plate formation at the MOR (Gurnis & Müller, 2003). Moreover, the depth anomaly displays a 'V'-shape, with the base of the 'V' aligned with the ridge and directed to the west (Gurnis & Müller, 2003). Kempton et al. (2001) explain the cause of this morphology to be due to the depth anomaly migrating west at rate of approximately 1.5cm/year. Further observations obtained from the residual depth anomalies include the symmetry of the AAD with respect to the SEIR and that the largest depth anomalies are found closer to the Australian-Antarctic margins, not on the ridge itself (Gurnis & Müller, 2003). The residual depth anomaly possess an hourglass appearance in which the width of the depth anomaly is both wider, in the east-west direction, and deeper towards the AAD, in comparison to the adjacent ridge crest (Figure 1.2; Gurnis & Müller, 2003). Figure 1.2 A residual topography image of the AAD showing the isotopic signature of the MOR ridge basalts in which red denotes the Indian mantle and yellow, the Pacific mantle (Gurnis et al., 1998). The research presented by Marks et al. (1991) is consistent with the idea that the mantle structure deepens with proximity to the centre of the AAD. They also describe the AAD to have a thick lithosphere, but thin crust below it, as well as a regional geoid low. Sea-Floor Morphology Anomalies The AAD is well known to possess extremely rough ridge-flank topography caused by asymmetric spreading (Figure 1.2 Forsyth et al., 1987; Palmer et al., 1992; Gurnis & Müller, 2003). Gurnis and Müller (2003) refer to this as chaotic topography, which is the domination of the sea-floor by irregular blocks of oceanic crust separated by irregular deep valleys. This is distinctly different to normal abyssal hill fabric. Marks and Stock (1995) attribute this chaotic topography, or crenulated geometry, to the formation of many spreading segments which have been offset by alternating transform faults that are orientated on a north-south axis. This crenulate geometry has evolved over the last 25 Ma and is driven by variable, asymmetric seafloor spreading and propagating rifts (Palmer et al., 1992). A deeper than normal axial valley is also characteristic of the AAD in which depths of approximately 800m are reached (Marks and Stock, 1995; Gurnis & Müller, 2003). Distinct Isotopic Provinces Through Strontium, Lead and Neodynium isotope analysis of basalts along the SEIR, two distinct isotopic provinces were distinguished, one to the west of the AAD, which is characteristic of the Indian Ocean, and the other to the east, which is characteristic of the Pacific Ocean (Figure 1.2). According to Pyle et al. (1992), this distinct isotopic boundary marks the convergence of the Indian and Pacific magmas, and appears to be migrating at a rate of 2.5cm/year. Mantle Anomalies Major element analysis of basalts from the AAD indicate that the mantle temperatures experienced within the AAD are lower than normal. Furthermore, the temperature and extent of melting below the AAD is lower in comparison with normal ridges (Gurnis et al., 1998). As a consequence of the cooler upper mantle, less melt is produced and therefore the crust is thinner within the AAD (~4.2km) compared with the surrounding SEIR crust (~7.2km; West et al., 1994; Gurnis et al., 1998). Sempéré et al. (1991) explain that the differences in the spreading characteristics, such as temperature of mantle material, is the basis for the differences experienced in magma supply and mantle structure. As a consequence of the uniqueness of the AAD, many theories on its origins have evolved. Download MS Word document of this page çBack•Nextè Home Lara Ainley: 0413554, lain2765@mail.usyd.edu.au Melanie de Leon: 0411604, mede4027@mail.usyd.edu.au Hannah Power: 0420262, hpow4369@mail.usyd.edu.au School of Geosciences, University of Sydney NSW 2006, Australia Last Updated: 02/06/2006

The Australian-Antarctic Didscordance (AAD) is defined as the deepest portion (~4-5km) of the global mid-ocean ridge (MOR) system (Gurnis et al., 1998). It is approximately 600km long and is a segment of the South East Indian Ridge (SEIR) south of Australia, between 115 and 128˚E (Figure 1.1; Gurnis & Müller, 2003).

Figure 1.1 Location of the Australian-Antarctic Discordance. The north-south trending crenulations can be seen (Source: http://www.geosci.usyd.edu.au/research/marinegeophysics/Resprojects/geo_figures.html).

The characteristics of the AAD include:

Depth Anomalies

The residual topography of the sea floor reveals a linear north-south-trending depression extending from the Great Australian Bight (GAB), through the region of the AAD, to the Antarctic margin, with residual topography values exceeding 1000 m (Figure 1.2; Gurnis et al. 1998; Gurnis & Müller, 2003). Gurnis et al. (1998) explain that in comparison with global residual oceanic topography, the AAD is a well-known bathymetric anomaly that is not associated with present-day subduction. This depth anomaly is symmetrical with respect to the SEIR which suggests that it is a property of plate formation at the MOR (Gurnis & Müller, 2003). Moreover, the depth anomaly displays a 'V'-shape, with the base of the 'V' aligned with the ridge and directed to the west (Gurnis & Müller, 2003). Kempton et al. (2001) explain the cause of this morphology to be due to the depth anomaly migrating west at rate of approximately 1.5cm/year.

Further observations obtained from the residual depth anomalies include the symmetry of the AAD with respect to the SEIR and that the largest depth anomalies are found closer to the Australian-Antarctic margins, not on the ridge itself (Gurnis & Müller, 2003). The residual depth anomaly possess an hourglass appearance in which the width of the depth anomaly is both wider, in the east-west direction, and deeper towards the AAD, in comparison to the adjacent ridge crest (Figure 1.2; Gurnis & Müller, 2003).

Figure 1.2 A residual topography image of the AAD showing the isotopic signature of the MOR ridge basalts in which red denotes the Indian mantle and yellow, the Pacific mantle (Gurnis et al., 1998).

The research presented by Marks et al. (1991) is consistent with the idea that the mantle structure deepens with proximity to the centre of the AAD. They also describe the AAD to have a thick lithosphere, but thin crust below it, as well as a regional geoid low.

Sea-Floor Morphology Anomalies

The AAD is well known to possess extremely rough ridge-flank topography caused by asymmetric spreading (Figure 1.2 Forsyth et al., 1987; Palmer et al., 1992; Gurnis & Müller, 2003). Gurnis and Müller (2003) refer to this as chaotic topography, which is the domination of the sea-floor by irregular blocks of oceanic crust separated by irregular deep valleys. This is distinctly different to normal abyssal hill fabric.

Marks and Stock (1995) attribute this chaotic topography, or crenulated geometry, to the formation of many spreading segments which have been offset by alternating transform faults that are orientated on a north-south axis. This crenulate geometry has evolved over the last 25 Ma and is driven by variable, asymmetric seafloor spreading and propagating rifts (Palmer et al., 1992).

A deeper than normal axial valley is also characteristic of the AAD in which depths of approximately 800m are reached (Marks and Stock, 1995; Gurnis & Müller, 2003).

Distinct Isotopic Provinces

Through Strontium, Lead and Neodynium isotope analysis of basalts along the SEIR, two distinct isotopic provinces were distinguished, one to the west of the AAD, which is characteristic of the Indian Ocean, and the other to the east, which is characteristic of the Pacific Ocean (Figure 1.2). According to Pyle et al. (1992), this distinct isotopic boundary marks the convergence of the Indian and Pacific magmas, and appears to be migrating at a rate of 2.5cm/year.

Mantle Anomalies

Major element analysis of basalts from the AAD indicate that the mantle temperatures experienced within the AAD are lower than normal. Furthermore, the temperature and extent of melting below the AAD is lower in comparison with normal ridges (Gurnis et al., 1998). As a consequence of the cooler upper mantle, less melt is produced and therefore the crust is thinner within the AAD (~4.2km) compared with the surrounding SEIR crust (~7.2km; West et al., 1994; Gurnis et al., 1998).

Sempéré et al. (1991) explain that the differences in the spreading characteristics, such as temperature of mantle material, is the basis for the differences experienced in magma supply and mantle structure.

As a consequence of the uniqueness of the AAD, many theories on its origins have evolved.

Download MS Word document of this page

çBack•Nextè

Home

Lara Ainley: 0413554, lain2765@mail.usyd.edu.au

Melanie de Leon: 0411604, mede4027@mail.usyd.edu.au
Hannah Power: 0420262, hpow4369@mail.usyd.edu.au

School of Geosciences, University of Sydney

NSW 2006, Australia

Last Updated: 02/06/2006